SQL As Understood By SQLite

BEGIN TRANSACTION

No changes can be made to the database except within a transaction.
Any command that changes the database (basically, any SQL command
other than SELECT) will automatically start a transaction if
one is not already in effect. Automatically started transactions
are committed when the last query finishes.

Transactions can be started manually using the BEGIN
command. Such transactions usually persist until the next
COMMIT or ROLLBACK command. But a transaction will also
ROLLBACK if the database is closed or if an error occurs
and the ROLLBACK conflict resolution algorithm is specified.
See the documentation on the ON CONFLICT
clause for additional information about the ROLLBACK
conflict resolution algorithm.

END TRANSACTION is an alias for COMMIT.

Transactions created using BEGIN...COMMIT do not nest.
For nested transactions, use the SAVEPOINT and RELEASE commands.
The "TO SAVEPOINT name" clause of the ROLLBACK command shown
in the syntax diagram above is only applicable to SAVEPOINT
transactions. An attempt to invoke the BEGIN command within
a transaction will fail with an error, regardless of whether
the transaction was started by SAVEPOINT or a prior BEGIN.
The COMMIT command and the ROLLBACK command without the TO clause
work the same on SAVEPOINT transactions as they do with transactions
started by BEGIN.

Transactions can be deferred, immediate, or exclusive.
The default transaction behavior is deferred.
Deferred means that no locks are acquired
on the database until the database is first accessed. Thus with a
deferred transaction, the BEGIN statement itself does nothing to the
filesystem. Locks
are not acquired until the first read or write operation. The first read
operation against a database creates a SHARED lock and the first
write operation creates a RESERVED lock. Because the acquisition of
locks is deferred until they are needed, it is possible that another
thread or process could create a separate transaction and write to
the database after the BEGIN on the current thread has executed.
If the transaction is immediate, then RESERVED locks
are acquired on all databases as soon as the BEGIN command is
executed, without waiting for the
database to be used. After a BEGIN IMMEDIATE,
no other database connection will be able to write to the database or
do a BEGIN IMMEDIATE or BEGIN EXCLUSIVE. Other processes can continue
to read from the database, however. An exclusive transaction causes
EXCLUSIVE locks to be acquired on all databases. After a BEGIN
EXCLUSIVE, no other database connection except for read_uncommitted
connections will be able to read the database and no other connection without
exception will be able to write the database until the transaction is
complete.

An implicit transaction (a transaction that is started automatically,
not a transaction started by BEGIN) is committed automatically when
the last active statement finishes. A statement finishes when its
prepared statement is reset or
finalized. An open sqlite3_blob used for
incremental BLOB I/O counts as an unfinished statement. The sqlite3_blob
finishes when it is closed.

The explicit COMMIT command runs immediately, even if there are
pending SELECT statements. However, if there are pending
write operations, the COMMIT command
will fail with an error code SQLITE_BUSY.

An attempt to execute COMMIT might also result in an SQLITE_BUSY return code
if an another thread or process has a shared lock on the database
that prevented the database from being updated. When COMMIT fails in this
way, the transaction remains active and the COMMIT can be retried later
after the reader has had a chance to clear.

In very old versions of SQLite (before version 3.7.11 - 2012-03-20)
the ROLLBACK will fail with an error code
SQLITE_BUSY if there are any pending queries. In more recent
versions of SQLite, the ROLLBACK will proceed and pending statements
will often be aborted, causing them to return an SQLITE_ABORT or
SQLITE_ABORT_ROLLBACK error.
In SQLite version 3.8.8 (2015-01-16) and later,
a pending read will continue functioning
after the ROLLBACK as long as the ROLLBACK does not modify the database
schema.

If PRAGMA journal_mode is set to OFF (thus disabling the rollback journal
file) then the behavior of the ROLLBACK command is undefined.

Response To Errors Within A Transaction

If certain kinds of errors occur within a transaction, the
transaction may or may not be rolled back automatically. The
errors that can cause an automatic rollback include:

For all of these errors, SQLite attempts to undo just the one statement
it was working on and leave changes from prior statements within the
same transaction intact and continue with the transaction. However,
depending on the statement being evaluated and the point at which the
error occurs, it might be necessary for SQLite to rollback and
cancel the entire transaction. An application can tell which
course of action SQLite took by using the
sqlite3_get_autocommit() C-language interface.

It is recommended that applications respond to the errors
listed above by explicitly issuing a ROLLBACK command. If the
transaction has already been rolled back automatically
by the error response, then the ROLLBACK command will fail with an
error, but no harm is caused by this.

Future versions of SQLite may extend the list of errors which
might cause automatic transaction rollback. Future versions of
SQLite might change the error response. In particular, we may
choose to simplify the interface in future versions of SQLite by
causing the errors above to force an unconditional rollback.